Short pulse lasers are built based on various techniques. One of the most popular techniques for making lasers with picosecond and sub-picosecond pulse durations is the mode-locking, where a short pulse is formed in the laser resonator in case a fixed phase relationship is achieved between its longitudinal modes, or more precisely, between the lines in the spectrum of the laser output. Typically such laser apparatus comprises a cavity with gain medium placed therein and an active or passive element, which creates resonator losses with a period of time, which corresponds to a round-trip time of a pulse inside said cavity. Passive elements are usually saturable absorbers, which change their transparency to a light pulse with increasing (decreasing) optical intensity.
Active mode locking is carried out by using acousto-optic, electro-optic modulators, a Mach-Zehnder integrated-optic modulator, or a semiconductor electro-absorption modulator. The modulation is synchronized with the resonator round trips and it usually results in generation of picosecond pulses.
Since saturable absorbers have fairly short recovery time, as compared to modulation frequency of active modulators, in general the achieved pulse durations are typically shorter for passively mode locked lasers.
Mode locked lasers usually feature high pulse repetition rates but low pulse energy. However if a higher pulse energy is desired, amplification techniques, such as cavity dumping or regenerative amplification are used.
Regenerative amplification (RA) is a powerful means for amplifying low energy pulses in a separate cavity. A pulse from mode locked laser is injected in a RA cavity by means of optical switch, comprised of an electro-optical modulator and a polarizer. Within several round-trips inside the RA cavity, the pulse is strongly amplified and out-coupled using the same or a different optical switch. This technique provides high amplification, however the optical design requires pricey components and is quite complex to build.
Cavity dumping is another amplification technique, where an optical switch extracts an amplified pulse from an optical cavity. The cavity can be operated both by mode locking or Q-switching principles and the optical switch is usually an acousto-optic modulator or an electro-optic modulator, such as Pockels Cell.
Cavity dumping is rarely used with actively mode locked lasers. Such technique is complicated as it involves using several optical modulators—one for active mode locking, the other for cavity dumping. For example, a Pockels Cell can be used for cavity dumping and an acousto-optic modulator could be arranged to create periodic losses inside the optical cavity or vice versa.
A U.S. Pat. No. 4,375,684 describes a technique where a single element interposed in a laser resonator cavity provides for AM mode-locking, Q-switching and dumping in a sequential operation. A Pockels cell or like polarization rotation device is first energized to prevent build-up of radiation in a laser cavity, is then switched to provide for mode-locking by energization with a periodically varying signal, and is finally energized to dump a single mode-locked pulse. The sequential operation permits the use of a single electro-optic element and a single pair of electrodes. Q-switching, mode-locking and dumping are accomplished utilizing the same physical effect in the crystal, e.g., polarization rotation.
The technique described above provides a solution, where a single electro-optic modulator can fully control the complete laser system. However the description and claims of the said patent provide an embodiment, where the Pockels Cell is energized from several different electrical sources and switching, i.e. multiplexing, is performed to transfer from one source (oscillating voltage) to another source (BIAS power supply). Such switching is complicated as very fast and expensive electrical switches should be used to control switching between the two sources. In addition, perfect phase matching should be kept between the two electrical regimes, which is also complicated.
U.S. Pat. No. 7,929,579 describes another improvement to the technique. Apparatuses and methods are disclosed for applying laser energy having desired pulse characteristics, including a sufficiently short duration and/or a sufficiently high energy for the photomechanical treatment of skin pigmentations and pigmented lesions, both naturally-occurring (e.g., birthmarks), as well as artificial (e.g., tattoos). The laser energy may be generated with an apparatus having a resonator with the capability of switching between a mode-locked pulse operating mode and an amplification operating mode. The operating modes are carried out through the application of a time-dependent bias voltage, having waveforms as described herein, to an electro-optical device positioned along the optical axis of the resonator.
This patent describes a method of controlling a Pockels Cell by means of switching circuitry, which provides switching between several power sources by means of transistors. However such system is also very complex and expensive to make in terms of electronics.